Synchronous rectification switching power supply

ABSTRACT

There is provided a flay-back type synchronous rectifying switching power supply device in which a rectifying element is surely turned off before the main switch is turned on even when the on-time of a main switch element is lengthened due to sudden variation of a load. The synchronous rectifying switching power supply device is equipped with a synchronous rectifying element (Q 2 ) connected to a secondary winding (N 2 ) of a transformer (T) in series and driving means comprising an auxiliary winding (N 3 ), etc. for turning on the synchronous rectifying element (Q 2 ) complementarily with a main switch element (Q 1 ) between output terminals ( 13, 14 ). A transistor (Tr 1 ) serving as interrupting means for turning off the synchronous rectifying element (Q 2 ) is provided between the gate and source of the synchronous rectifying element (Q 2 ). An off-timing at which the synchronous rectifying element (Q 2 ) is turned off by the interrupting means (Tr 1 ) is set within a timing range which corresponds to a fixed time after the main switch element (Q 1 ) is turned on and also is as near as a fixed driving period of the main switch element (Q 1 ).

TECHNICAL FIELD

The present invention relates to a synchronous rectifying type switchingpower supply device for converting a DC voltage to a desired voltage andsupplying the voltage to electronic equipment, and particularly to afly-back type synchronous rectifying switching power supply device.

BACKGROUND ART

In a conventional fly-back converter which is a switching power supplydevice having a synchronous rectification type rectifying circuit, aseries circuit comprising a DC power source and a main switching elementis connected to a primary winding side of a transformer, and asynchronous rectifying element is provided to a secondary winding of thetransformer in series, and further connected to an output terminalthrough a rectifying circuit as disclosed in JP-A-2000-116122. Thefly-back converter controls to turn on/off the main switch element ofMOS-FET. When the main switch element is turned off, MOS-FET which is asynchronous rectifying element of the secondary side circuit of thetransformer is turned on, and an output capacitor of the rectifyingcircuit is charged by a fly-back voltage occurring at the secondarywinding. Thereafter, the synchronous rectifying element is turned offbefore the main switch element is turned on, and this operation isrepeated to supply power to the output side.

In the case of the synchronous rectifying type fly-back converter asdescribed above, when the off-timing of the synchronous rectifyingelement is missed and thus the main switch is turned on under the statethat the synchronous rectifying element is still turned on, the circuitat the secondary side of the power supply device falls into ashort-circuited state, and thus large surge current flows into the mainswitch element, so that the main switch element, the synchronousrectifying element, etc. may be broken.

Therefore, as disclosed in JP-A-2000-116122, there is known a switchingpower supply circuit in which in order to prevent the simultaneouson-state of the main switch element and the synchronous rectifyingelement, in response to turn-off the main switch element, a timingcapacitor is charged with current determined by a voltage induced at anauxiliary winding and a timing resistor after the synchronous rectifyingelement is turned on, and an auxiliary transistor is turned on after afixed time to turn off the rectifying element.

In the case of the synchronous rectifying type switching power supplydevice of the prior art described above, the fixed time Tc of the timingcapacitor is set to be prior to the on-timing of the main switch elementto some degree by providing a dead time td so that the rectifyingelement is necessarily turned off before the main switch element isturned on as shown in FIG. 1. The dead time td is set by the timeconstant of the timing capacitor so that the rectifying element isturned off within an off-time of the main switch element which isdetermined by an input voltage, an output voltage and a turn ratio ofthe transformer.

However, in the case of the conventional synchronous rectifying typefly-back converter, there is a case where load current increases rapidlyand the main switch element is turned on for a time longer than theon-time of the main switch element determined by the input/outputvoltage and the turn ratio of the transformer. In such a case, there isa case where the voltage of the timing capacitor does not reach thethreshold voltage of the auxiliary transistor element for turning offthe synchronous rectifying element within a fixed on/off period of themain switch element as indicated by a broken line of FIG. 1. In such acase, there is a problem that the main switch element is turned onbefore the rectifying element is turned off, and very large surgecurrent flows into the main switch element, so that the circuit at thesecondary side of the power supply device is kept short-circuited andthus through current flows into the secondary side circuit, resulting inbreaking of the main switch element, the synchronous rectifying element,etc.

During the period of the dead time td, the rectifying operation iscarried out by a diode connected to the synchronous rectifying elementin parallel or a body diode of MOS-FET serving as the synchronousrectifying element. The loss during the rectifying period of the diodeis larger than that during the period when the rectifying element ofMOS-FET is turned on. Accordingly, the dead time is required to be asshort as possible, however, there is a problem that it is impossible toshorten the dead time td in order to surely turn off the synchronousrectifying element before the main switch element is turned on.Furthermore, the switching frequency cannot be increased because thedead time td cannot be shortened, which also prevents miniaturizationand cost-down of the device.

In addition, there is also a problem that when a voltage not less than aset voltage is applied between the output voltage terminals from theexternal or when power supply is stopped in a case where alarge-capacity external capacitor is provided between the outputterminals, the synchronous rectifying element at the secondary sidecannot be turned off and thus through current flows, or self-excitationoscillation occurs in the secondary side circuit by the power of thelarge-capacity external capacitor connected between the outputterminals.

The invention has an object to provide a fly-back type synchronousrectifying switching power supply device which can surely turn off asynchronous rectifying element before a main switch is turned on evenwhen the on-time of the main switch element is lengthened due to suddenvariation of a load.

Furthermore, the invention has an object to provide a fly-back typesynchronous rectifying switching power supply device which can preventthrough current of a switching power supply circuit and self-excitationoscillation irrespective of sudden variation of a load or an externalequipment connected between output terminals.

DISCLOSURE OF THE INVENTION

A fly-back type synchronous rectifying switching power supply device inwhich a primary winding of a transformer and a main switch element areconnected to each other in series between input terminals and which hasa control circuit for subjecting the main switch element to PWM controlwithin a fixed period, a synchronous rectifying element connected to asecondary winding of the transformer in series between output terminals,and driving means for turning on the synchronous rectifying elementcomplementarily with the switching element, is equipped with a differentpower supply source charged by a pulse voltage occurring at thesecondary side winding of the transformer through a switching operationof the main switch element, and interrupting means which is disposedbetween the gate and source of the synchronous rectifying element andturns off the synchronous rectifying element, wherein an off-timing atwhich the interrupting means turns off the synchronous rectifyingelement is set to a timing which corresponds to a fixed time set bycurrent from the different power source after the switch element isturned on and is within a range which is as near as a fixed drivingperiod of the switch element.

The interrupting means comprises a transistor and a timing capacitorconnected to a signal input terminal of the transistor, the timingcapacitor is charged by the different power source and discharged at theinstantaneous time when the main transistor is turned on, the timingcapacitor is started to be charged from the instantaneous timeconcerned, and a period from this time to time in which the voltage ofthe timing capacitor exceeds a threshold value of the signal inputterminal of the transistor is set to a time within the fixed drivingperiod of the switch element.

The different power source is a constant voltage source or constantcurrent source connected to the secondary side of the transformer. Asnubber circuit for absorbing surge energy when the synchronousrectifying element is turned off may be also used as the different powersource for charging the timing capacitor so that the timing capacitor ischarged with the energy absorbed by the snubber circuit.

According to the synchronous rectifying switching power source of theinvention, the synchronous rectifying element is surely turned offwithin a fixed period from the on-timing of the main switch element, sothat the main switch element and the synchronous rectifying element canbe prevented from simultaneously falling into an on-state even when loadcurrent varies suddenly. Accordingly, the dead time from the turn-offtime of the synchronous rectifying element until the on-timing of themain switch can be shortened as much as possible, the rectifying periodbased on the diode is reduced to suppress the loss, and the switchingfrequency can be increased. Furthermore, the invention contributes tominiaturization and reduction in cost.

Furthermore, a fly-back type synchronous rectifying switching powersupply device in which a primary winding of a transformer and a mainswitch element are connected to each other in series between inputterminals and which has a control circuit for subjecting the main switchelement to PWM control within a fixed period, a synchronous rectifyingelement connected to a secondary winding of the transformer in seriesbetween output terminals, and driving means for turning on thesynchronous rectifying element complementarily with the switchingelement, is equipped with a different power supply source charged by apulse voltage occurring at the secondary side winding of the transformerthrough a switching operation of the main switch element, interruptingmeans which is disposed between the gate and source of the synchronousrectifying element and turns off the synchronous rectifying element, anda control element for comparing the output voltage of the differentpower source with the output voltage of the output terminal of theswitching power supply device, and controlling the interrupting means toturn off the synchronous rectifying element when the output voltage ofthe different power source is reduced to a fixed value or less.

A transistor of the interrupting means is an npn transistor for turningoff the synchronous rectifying element, and the control elementcomprises a pnp transistor whose emitter and collector are connected tothe output terminal and the base of the npn transistor respectively, andthe output of the different power source is connected to the base of thepnp transistor. The output voltage of the different power source may bedivided and input a divided voltage to the base of the pnp transistor.

According to the other invention of this application, through currentdue to sudden variation of a load and a self-excitation oscillationphenomenon occurring when the power source is stopped or an externalvoltage is applied can be surely prevented, the miniaturization of theconstituent parts of the device can be performed and the miniaturizationof the whole device and the reduction in cost can be enabled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a timing chart showing the operation of a conventionalfly-back type synchronous rectifying switching power supply device.

FIG. 2 is a schematic circuit diagram showing a synchronous rectifyingswitching power supply device according to an embodiment of theinvention.

FIG. 3 is a timing chart (A) showing the operation when a duty ratio ofthe main switch element of the synchronous rectifying switching powersupply device of this embodiment is broad, and a timing chart (B)showing the operation when the duty ratio is narrow.

FIG. 4 is a schematic circuit diagram showing a synchronous rectifyingswitching power supply device according to a second embodiment of theinvention.

FIG. 5 is a schematic circuit diagram showing a synchronous rectifyingswitching power supply device according to a third embodiment of theinvention.

FIG. 6 is a schematic circuit diagram showing a synchronous rectifyingswitching power supply device according to a fourth embodiment of theinvention.

FIG. 7 is a schematic circuit diagram showing a synchronous rectifyingswitching power supply device according to a fifth embodiment of theinvention.

FIG. 8 is a schematic circuit diagram showing another example of thesynchronous rectifying switching power supply device of the fifthembodiment according to the invention.

BEST MODES FOR CARRYING OUT THE INVENTION

Embodiments of the invention will be described with reference to thedrawings. FIG. 2 is a circuit showing a fly-back type synchronousrectifying switching power device according to a first embodiment of theinvention. In the switching power supply circuit, a DC power source 10is connected between input terminals 11 and 12, and a main switchelement Q1 of MOS-FET is connected to the primary winding N1 of thetransformer T in series. The input terminal 11 at the plus side of theDC power source 10 is connected to one dot-affixed side terminal of theprimary winding N1 which is a terminal at which a positive voltageoccurs when the main switching element Q1 is turned on, and the othernon-dot-affixed side terminal of the transformer T is connected to thedrain of the main switch element Q1. The source of the main switchelement Q1 is connected to the input terminal 12 at the minus side ofthe DC power source 10, and the gate of the main switch element Q1 isconnected to a driving signal output of a control circuit 18 forsubjecting the main switch element Q1 to PWM (Pulse width Modulation)control in accordance with an input/output condition at a fixed period.

A non-dot-affixed side terminal of the secondary winding N2 of thetransformer T is connected to one end of an output capacitor C2, and theother dot-affixed side terminal of the secondary winding N2 of thetransformer T is connected to the drain of the synchronous rectifyingelement Q2 of MOS-FET. The source of the synchronous rectifying elementQ2 is connected to a reference potential side corresponding to the otherend side of the output capacitor C2. Both the ends of the outputcapacitor C2 are connected to the output terminals 13 and 14. A diode D4is connected between the drain and source of the synchronous rectifyingelement Q2 in parallel. The anode and cathode of the diode D4 areconnected to the source and drain of the synchronous rectifying elementQ2, respectively. Accordingly, the diode D4 may be replaced by a bodydiode of the synchronous rectifying element Q2 of MOS-FET.

Furthermore, an auxiliary winding N3 is provided as driving means of thesynchronous rectifying element Q2 at the secondary side of thetransformer T, a dot-affixed side terminal of the auxiliary winding N3is connected to reference potential, and a non-dot-affixed side terminalis connected to one end of an operation accelerating capacitor C4through a resistor R1. The other end of the capacitor C4 is connected tothe cathode of the diode D1, and the anode of the diode D1 is connectedto the reference potential. The gate of the synchronous rectifyingelement Q2 is connected between the cathode of the diode D1 and theother end of the capacitor C4.

The collector of an npn type transistor Tr1 is connected to the gate ofthe synchronous rectifying element Q2, and the emitter of the transistorTr1 is connected to the reference potential. One end of a timingcapacitor C3 is connected to the base serving as a signal input signalterminal of the transistor Tr1, and the other end of the timingcapacitor C3 is connected to the reference potential. The output of aconstant voltage source 16 which is a different power source isconnected to the base of the transistor Tr1 through a resistor R2, andalso connected to the collector of an npn type transistor Tr2. Theemitter of the transistor Tr2 is connected to the reference potential,and the base thereof is connected to the dot-affixed side terminal ofthe secondary winding N2 through a capacitor C7. A resistor R3 and adiode D2 are connected in parallel between the base and emitter of thetransistor Tr2. The cathode and anode of the diode D2 are connected tothe base and the reference potential, respectively.

The constant voltage source 16 comprises a capacitor C5 whose one end isconnected to a dot-affixed side terminal of the secondary winding N2, adiode D3 whose anode is connected to the other end of the capacitor C5,a capacitor C6 connected between the cathode of the diode D3 and thereference potential, and a zener diode ZD1 connected between the anodeof the diode D3 and the reference potential. The cathode of the zenerdiode ZD1 is connected to the anode of the diode D3, and the anodethereof is connected to the reference potential. The constant voltagesource 16 also serves as a snubber circuit for absorbing surge energywhen the synchronous rectifying element Q2 is turned off.

Next, the control method and operation of the synchronous rectifyingswitching power supply of this embodiment will be described withreference to FIGS. 2 and 3. First, when the main switch element Q1 isturned on in the circuit of FIG. 2, each of the dot-affixed sides of theprimary winding N1 and the secondary winding N2 is set to plus. However,as shown in FIGS. 3(A), (B), the potential Vgs between the gate andsource of the synchronous rectifying element Q2 is low, and thesynchronous rectifying element Q2 is turned off, so that no current Id1flows in the synchronous rectifying element Q2. Furthermore, at thistime, current flows from the dot-affixed side of the secondary windingN2 into the constant voltage source 16 to charge the capacitors C5 andC6, and a constant voltage set by the zener diode ZD1 is achieved at oneend of the capacitor C6 of the constant voltage source 16. Current flowsfrom one end of the capacitor C6 corresponding to the output of theconstant voltage source 16 through the resistor R2 into the timingcapacitor C3 to charge the timing capacitor C3. Furthermore, during theperiod when the main switch element Q1 is turned on, the dot-affixedside of the auxiliary winding N3 is high, however, the gate of thesynchronous rectifying element Q2 is set to the reference potentialthrough the diode D1.

Thereafter, when the main switch element Q1 is turned off on the basisof the PWM control in conformity with an input/output condition by thecontrol circuit 18, a fly-back voltage occurs at the non-dot-affixedside of the secondary winding N2, and also a fly-back voltagesimultaneously occurs at the non-dot-affixed side of the auxiliarywinding N3, so that the gate capacitor Ciss of the synchronousrectifying element Q2 is charged through the capacitor C4 and thepotential Vgs between the gate and the source is made high, therebyturning on the synchronous rectifying element Q2. Accordingly, thecurrent Id1 flows from the non-dot-affixed side terminal of thesecondary winding through the output capacitor C2 to the dot-affixedside terminal to charge the output capacitor C2.

Furthermore, the timing capacitor C3 is charged with current from theconstant voltage source 16 immediately after the main switch element Q1is turned on, and when a fixed time elapses, the potential of the timingcapacitor C3 reaches the threshold value of the base of the transistorTr1. Accordingly, the transistor Tr1 is turned on, the gate capacitorCiss of the synchronous rectifying element Q2 is discharged, and thesynchronous rectifying element Q2 is turned off. However, even afterthat, current Id2 is made to flow by the diode D4 equipped in parallelto the synchronous rectifying element Q2 until the main switch elementQ1 is turned on. The current Id2 is smaller than the current Id1 becausethe diode imposes some loss. The period during which current is made toflow by the diode D4 corresponds to the dead time dt for turning off thesynchronous rectifying element Q2 before the main switch element Q1 isturned on.

When the main switch Q1 is turned on, the potential of the dot-affixedside of the secondary winding N2 is applied to the base of thetransistor Tr1 through the capacitor C7, and at that instantaneous timethe transistor Tr1 is made low, and the charges of the timing capacitorC3 are instantaneously discharged. This period is completed in asufficiently shorter instantaneous time as compared with the switchingfrequency of the main switch element Q1 because the capacitance of thecapacitor C7 is relatively sufficiently small. From this instantaneoustime, the charging of the timing capacitor C3 is started again asdescribed above.

In the fly-back type synchronous rectifying switching power supplydevice of this embodiment, the switching frequency T of the main switchelement Q1 is fixed by the control circuit 18, and Duty corresponding tothe on-period of the main switch element Q1 is varied in conformity withthe input/output condition as shown in FIGS. 3(A), (B). However, thepotential of the timing capacitor C3 applied to the base of thetransistor Tr1 reaches the threshold value of the base of the transistorTr1 in a fixed time from the on-timing of the main switch element Q1 bythe constant voltage source 16. Therefore, since the output voltage isincreased when the load current rapidly increases and the output voltageis transitionally lowered, the synchronous rectifying element Q2 issurely turned off in a fixed time from the on-timing of the main switchelement Q1 even when the on-period of the main switch element Q istemporarily long. Accordingly, the dead time td from the turn-off of thesynchronous rectifying element Q2 until the turn-on of the main switchelement Q1 can be set to be as short as possible, the rectifying periodby the diode D4 is shortened to suppress the loss, and the switchingfrequency can be increased.

Furthermore, the constant voltage source 16 serving as the chargingcircuit for the timing capacitor C3 constitutes the snubber circuit forabsorbing the surge energy when the synchronous rectifying element Q2 isturned off, and the timing capacitor is charged with the energy absorbedby the snubber circuit. Therefore, the constant voltage source 16 canserve as a power source having a high energy efficiency.

Next, a synchronous rectifying switching power supply device of a secondembodiment of the invention is shown in FIG. 4. Here, the sameconstituent elements as the above embodiment are represented by the samereference numerals, and the description thereof is omitted. Thisembodiment is different from the first embodiment in that the output ofa constant current source 20 comprising a constant current circuit isconnected to the timing capacitor C3.

The constant current source 20 is equipped with a diode D5 whose anodeis connected to a dot-affixed side terminal of the secondary winding N2of the transformer T, and a capacitor C8 having one end connected to thecathode of the diode D5 and the other end connected to the referencepotential. Furthermore, the cathode of the diode D5 is connected to theemitter of an pnp type transistor Tr3 through a resistor R4, and thecollector of the transistor Tr3 is connected to one end of the timingcapacitor C3 as an output of the constant current source 20.Furthermore, the cathode of a zener diode ZD2 is connected to thecathode of the diode D5, and the anode of the zener diode ZD2 isconnected to the base of the transistor Tr3 and also connected to thereference potential through a resistor R5. The constant current is setby the resistor R4 and the constant voltage set by the zener diode ZD2.

In the fly-back type synchronous rectifying switching power supplydevice of this embodiment, the timing capacitor C3 can be charged withconstant current from the constant current circuit 20, and the voltageof the timing capacitor C3 is linearly increased.

The synchronous rectifying switching power supply device of thisembodiment can also achieve the same effect as the above embodiment. Inthis case, particularly the voltage of the timing capacitor C3 linearlyrises up, and it is easy to set the off-timing of the synchronousrectifying element Q2. A snubber circuit may be equipped with theconstant current source 20. Accordingly, the energy efficiency can befurther enhanced.

Next, a synchronous rectifying switching power supply device of a thirdembodiment of the invention is shown in FIG. 5. Here, the sameconstituent elements as the above embodiment are represented by the samereference numerals, and the description thereof is omitted. Thisembodiment is different from the first embodiment in that one end of thetiming capacitor C3 is connected between an operation acceleratingcapacitor C4 and the terminal connected to the collector of thetransistor Tr1 through a diode D6. The anode of the diode D6 isconnected to the timing capacitor C3 and the cathode thereof isconnected to the terminal of the capacitor C4.

The operation of the synchronous rectifying switching power supplydevice of this embodiment is the same as the circuit of the aboveembodiment, however, the discharge of the timing capacitor C3 is carriedout when the main switch element Q1 is turned off and the dot-affixedside of the auxiliary winding N3 is set to plus potential. At this time,current flows from the dot-affixed side terminal of the auxiliarywinding N3 through the electrode of the capacitor C3 at the referencepotential side and the electrode thereof at the opposite side, the diodeD6 and the capacitor C4 to the non-dot affixed side terminal of theauxiliary winding N3, thereby discharging the capacitor C3.

This embodiment can also achieve the same effect as the firstembodiment. Furthermore, the circuit construction for discharging thetiming capacitor C3 can be simplified, and the number of parts isreduced to promote miniaturization of the device and reduction of thecost.

Next, a synchronous rectifying switching power supply device accordingto a fourth embodiment of the invention is shown in FIG. 6. Here, theconstituent elements as the above embodiment are represented by the samereference numerals, and the description thereof is omitted. In thisembodiment, the constant voltage source 16 of the third embodiment isreplaced by the constant current source 20. According to thisembodiment, the same effect as the second embodiment can be achieved,and further the circuit construction for discharging the timingcapacitor C3 can be simplified as in the case of the third embodiment.Therefore, the miniaturization of the device and the reduction in costcan be promoted.

Next, a synchronous rectifying switching supply device of a fifthembodiment is shown in FIG. 7. Here, the same constituent elements asthe above embodiment are represented by the same reference numerals, andthe description thereof is omitted. In the synchronous rectifyingswitching power supply device of this embodiment, an input capacitor C1is equipped in parallel to the input terminals 11, 12 of the DC powersource, and a series circuit comprising the primary winding N1 of thetransformer T and the main switch element Q1 is connected to both theends of the input capacitor C1. The dot-affixed side of the primarywinding N1 of the transformer T is connected to the input terminal 11,and the non-dot-affixed side thereof is connected to the main switch Q1.The main switch element Q1 comprises a semiconductor switch element suchas MOS-FET. The non-dot-affixed side terminal of the secondary windingN2 of the transformer T is connected to the output terminal 13, and thedot-affixed side terminal thereof is connected to the synchronousrectifying element Q2 such as MOS-FET in series, and connected to theoutput terminal 14 at the reference potential side. Furthermore, asmoothing output capacitor C2 is equipped between the output terminals13 and 14.

A constant voltage source 16 corresponding to a different power sourceequipped at the secondary side of the power source circuit is providedbetween the drain and source of the synchronous rectifying element Q2.The constant voltage source 16 has a capacitor C5 which has one endconnected to the drain of the synchronous rectifying element Q2 ofMOS-FET and the other end connected to one end of a resistor R6. Theother end of the resistor R6 is connected to the cathode of the zenerdiode ZD1, and the anode of the zener diode ZD1 is connected to thereference potential. Furthermore, the other end of the resistor R6 isconnected to the anode of the diode D3, and the cathode of the diode D3is connected to the reference potential through a capacitor C6. Thepoint between the cathode of the diode D3 and the capacitor C6corresponds to the output of the constant voltage source 16.

Furthermore, the auxiliary winding N3 as the driving means of thesynchronous rectifying element Q2 is equipped at the secondary side ofthe transformer T, the dot-affixed side terminal of the auxiliarywinding N3 is connected to the reference potential, and thenon-dot-affixed side terminal thereof is connected to the gate of thesynchronous rectifying element Q2 through a speed-up capacitor C4. Thenon-dot-affixed side terminal of the auxiliary winding 3 is connected tothe cathode of the diode D6, and the anode of the diode D6 is connectedthrough the series circuit of a resistor R7 and a timing capacitor C3 tothe reference potential. The point between the resistor R7 and thetiming capacitor C3 is connected to the point between the cathode of thediode D3 and the capacitor C6 of the constant voltage source 16 througha resistor R2. Furthermore, the point between the resistor R7 and thecapacitor C3 is connected to the base of the transistor Tr1. Thecollector of the transistor Tr1 is connected to the gate of thesynchronous rectifying element Q2, and the emitter thereof is connectedto the reference potential. The cathode of the diode D1 is connected tothe gate of the synchronous rectifying element Q2, and the anode of thediode D1 is connected to the reference potential.

An npn type transistor Tr4 serving as switch element control means isconnected between the output terminal 13 and the base of the transistorTr1. The emitter of the transistor Tr4 is connected to the outputterminal 13, and the collector thereof is connected to the base of thetransistor Tr1 through a resistor R8. The base of the transistor Tr4 isconnected to the cathode of the diode D3 serving as the output of theconstant voltage source 16 and a capacitor C6.

In the operation of the switching power supply device, the main switchelement Q1 is turned on/off by the control circuit 18 to carry out PWMcontrol. During the on-period of the main switch element Q1, thesynchronous rectifying element Q2 is turned off, so that no currentflows, and the capacitor C6 of the constant voltage source 16 is chargedthrough the capacitor C5. The capacitor C5 restricts the charging amountof the capacitor C6. The charge voltage of the capacitor C6 is a voltageset by the zener diode ZD1. During the on-period of the main switchelement Q1, the capacitor C4 is charged by setting the cathode side ofthe diode D1 to plus by the auxiliary winding N3.

When the main switch element Q1 is turned off, the voltage at thenon-dot-affixed side terminal of the auxiliary winding N3 and thecharged voltage of the capacitor C4 are applied to the gate of thesynchronous rectifying element Q2 to charge the synchronous rectifyingelement Q2, so that the synchronous rectifying element Q2 is turned on.At the same time, the energy accumulated in the secondary winding N2 ischarged in the output capacitor C2 by the fly-back voltage occurring atthe secondary winding N2.

At the same time when the main switch element Q1 is turned off, thecharging of the timing capacitor C3 is started through the resistor R2by the constant voltage source 16. Accordingly, the potential of thetiming capacitor C3 is gradually increased, and when it reaches thepotential at which the transistor TR1 is turned on, the transistor TR1is turned on and the charges of the gate of the synchronous rectifyingelement Q2 are discharges, thereby turning off the synchronousrectifying element Q2. The timing at which the transistor TR1 is turnedon is set to a timing just before the main switch element Q1 is turnedon. When the main switch element Q1 is turned on, the charging of thecapacitor C6 of the constant voltage source 16 is started again, and thetiming capacitor C3 is discharged through the resistor R7 and the diodeD6.

Here, the potential of the capacitor C6 of the constant voltage source16 is applied to the base of the transistor Tr4, the output voltage ofthe constant voltage source 16 and the output voltage of the outputterminal 13 are compared with each other, the main switch element Q1 isstopped and the voltage of the capacitor C6 of the constant voltagesource 16 is reduced. When the transistor Tr4 is reduced to apredetermined potential or less at which the transistor Tr4 is turnedon, the pnp type transistor Tr4 is turned on, the timing capacitor C3 ischarged through the resistor R8 and thus the transistor Tr1 is turnedon. Accordingly, the gate charges of the synchronous rectifying elementQ2 are discharged, and the synchronous rectifying element Q2 is turnedoff. During the period when the transistor Tr4 is turned on, that is,during the period when the potential of the capacitor C6 of the constantvoltage source 16 is not more than a predetermined potential because ofstop of the main switch element Q1 or the like, the transistor tr1 isturned on, and the synchronous rectifying element Q2 is turned off. Whenthe main switch element Q1 starts its switching during the above period,the synchronous rectifying element Q2 carries out rectification by thebody diode thereof. Furthermore, the charging to the constant voltagesource 16 is carried out through the transistor Tr4, and the constantvoltage source 16 is quickly charged.

According to the switching power supply deice of this embodiment, evenwhen the main switch element Q1 is stopped due to sudden variation ofthe load and then the switching is re-started, the synchronousrectifying element Q2 is surely turned off, the output voltage of theconstant voltage source 16 is equal to a predetermined value or more,and the synchronous rectifying element Q2 is prevented from being turnedon until the synchronous rectifying element Q2 is allowed to be normallyand surely driven by the timing capacitor C3 and the transistor Tr1.Accordingly, no through current flows into the power supply circuit, andthe loss, etc. of the circuit elements can be surely prevented.

Even when a voltage higher than a set voltage is applied between theoutput terminals 13 and 14 by an external device, the main switchelement Q1 is stopped, and the output voltage of the constant voltage isreduced. In this case, the transistor Tr4 is likewise turned on due toreduction of the potential of the constant voltage source 16, and thesynchronous rectifying element Q2 is set to the off-state by thetransistor Tr1 to prevent self-excitation oscillation.

Furthermore, even when the main switch element Q1 stops its operation bya remote controller or interrupting the input voltage while alarge-capacity capacitor as an external device is connected between theoutput terminals 13 and 14, the voltage of the constant voltage source16 at the secondary side is reduced, and the synchronous rectifyingtransistor Q2 is turned off, thereby preventing the self-excitationoscillation due to the energy accumulated in the external large-capacitycapacitor between the output terminals 13 and 14. Furthermore, theenergy accumulated in the large-capacitance capacitor is consumed in theresistor R4, and the output voltage is quickly reduced.

The fly-back type synchronous rectifying switching power supply deviceof this invention is not limited to the above embodiment. For example,as shown in FIG. 8, the output potential of the constant voltage source16 of the circuit shown in FIG. 7 may be divided by the resistors R9,R10 to input the divided voltage to the base of the transistor Tr4,whereby the turn-on potential of the transistor Tr4 can be set to anyvalue by properly setting the resistors R9, R10. Furthermore, thiscircuit may be properly combined with other circuits.

Furthermore, in the fly-back type synchronous rectifying switching powersupply device of this embodiment, the charging of a different powersource may be carried out by an auxiliary winding, and the circuit maybe properly combined with other circuits.

1. A fly-back type synchronous rectifying switching power supply devicein which a primary winding of a transformer and a main switch elementare connected to each other in series between input terminals and whichhas a control circuit for subjecting the main switch element to PWMcontrol within a fixed period, a synchronous rectifying elementconnected to a secondary winding of the transformer in series betweenoutput terminals, and driving means for turning on the synchronousrectifying element complementarily with the switching element,characterized by further comprising a different power supply sourcecharged by a pulse voltage occurring at the secondary side winding ofthe transformer through a switching operation of the main switchelement, and interrupting means which is disposed between the gate andsource of the synchronous rectifying element and turns off thesynchronous rectifying element, wherein an off-timing at which theinterrupting means turns off the synchronous rectifying element is setto a timing which corresponds to a fixed time set by current from thedifferent power source after the switch element is turned on and iswithin a range which is as near as a fixed driving period of the switchelement.
 2. The synchronous rectifying switching power supply deviceaccording to claim 1, wherein the interrupting means comprises atransistor and a timing capacitor connected to a signal input terminalof the transistor, the timing capacitor is charged by the differentpower source and discharged at the instantaneous time when the maintransistor is turned on, the timing capacitor is started to be chargedfrom the instantaneous time concerned, and a period from this time to atime in which the voltage of the timing capacitor exceeds a thresholdvalue of the signal input terminal of the transistor is set to a timewithin the fixed driving period of the switch element.
 3. Thesynchronous rectifying switching power supply device according to claim2, wherein the different power source is a constant voltage source orconstant current source connected to the secondary side of thetransformer.
 4. The synchronous rectifying switching power supply deviceaccording to claim 3, wherein a snubber circuit for absorbing surgeenergy when the synchronous rectifying element is turned off is providedto the different power source for charging the timing capacitor so thatthe timing capacitor is charged with the energy absorbed by the snubbercircuit.
 5. A fly-back type synchronous rectifying switching powersupply device in which a primary winding of a transformer and a mainswitch element are connected to each other in series between inputterminals and which has a control circuit for subjecting the main switchelement to PWM control within a fixed period, a synchronous rectifyingelement connected to a secondary winding of the transformer in seriesbetween output terminals, and driving means for turning on thesynchronous rectifying element complementarily with the switchingelement, characterized by further comprising a different power supplysource charged by a pulse voltage occurring at the secondary sidewinding of the transformer through a switching operation of the mainswitch element, interrupting means which is disposed between the gateand source of the synchronous rectifying element and turns off thesynchronous rectifying element, and a control element for comparing theoutput voltage of the different power source with the output voltage ofthe output terminal of the switching power supply device, andcontrolling the interrupting means to turn off the synchronousrectifying element when the output voltage of the different power sourceis reduced to a fixed value or less.
 6. The synchronous rectifyingswitching power supply device according to claim 5, wherein a transistorof the interrupting means is an npn transistor for turning off thesynchronous rectifying element, and the control element comprises a pnptransistor whose emitter and collector are connected to the outputterminal and the base of the npn transistor respectively, and the outputof the different power source is connected to the base of the pnptransistor.
 7. The synchronous rectifying switching power supply deviceaccording to claim 6, wherein the different power source is a constantvoltage source connected to the secondary side of the transformer, andan output voltage of the different power source is divided to input thedivided voltage to the base of the pnp transistor.
 8. The synchronousrectifying switching power supply device according to claim 7, wherein asnubber circuit for absorbing surge energy when the synchronousrectifying element is turned off is provided to the different powersource for charging the timing capacitor, and the timing capacitor ischarged with the energy absorbed by the snubber circuit.